The desirability of increasing the bulk or volume of tobacco has long been recognized. Among the various reasons for desiring such increase in bulk or volume are compensation for the weight loss caused by the curing process. Increase in filling power permits the use of smaller amounts of tobacco in the production of firm cigarette rods or the like and results in a lower delivery of tar and nicotine than a comparable product made of unexpanded tobacco.
Different methods have been suggested in the art for expanding the volume of tobacco. For example, in U.S. Pat. No. 1,789,435 to Hawkins, tobacco is expanded by subjecting tobacco to a gas under pressure, which causes the gas to penetrate the constituents of the tobacco. Thereafter, the pressure is suddenly released whereupon the gas trapped in the tobacco expands. Among the gases mentioned as usable in the process are air, carbon dioxide and steam. The gas may be heated to facilitate the process.
According to the method of British Pat. No. 1,331,640, tobacco is impregnated with a compound capable of liberating a gas under conditions which do not impair the quality of tobacco. Nongaseous chemical compounds which liberate gases, such as carbon dioxide, nitrogen, oxygen and ammonia, upon thermal decomposition may be employed. Preferred compounds are those which decompose at relatively low temperatures including ammonium carbonates, ammonium carbamates, organic dicarboxylic acids and peroxides.
Tobacco expansion processes for enhancing the utility of tobacco stems for use in tobacco products are described in U.S. Pat. Nos. 3,409,022, 3,409,023, 3,409,027 and 3,409,028 all to Burde. According to the processes descirbed, tobacco stems are expanded by means of radiant heat or microwave energy. A method of reducing post-expansion shrinkage of tobacco stems and a method of preparing reconstituted tobacco sheets from puffed tobacco are also described. A further enchancement of the puffed product obtained from tobacco stems is described in U.S. Pat. No. 3,425,425 to Hind. In this patent, tobacco stems are treated with a solution of water-soluble carbohydrate prior to expansion.
In U.S. Pat. No. 3,771,533 to Armstrong et al., the various disadvantages of these and other prior art processes of tobacco expansion are noted. For example, some of the methods involve only moderate expansion, are not effective on tobacco leaf as well as tobacco stem, require elaborate and expensive equipment and/or involve introduction of foreign materials into the tobacco.
According to the expansion method of the Armstrong et al. patent, tobacco is treated with liquid or gaseous ammonia or with ammonium hydroxide or a combination of ammonia and carbon dioxide followed by heating to temperatures of from 250.degree. F. to 700.degree. F. for a time sufficient to puff the tobacco. Carbon dioxide may be added before, during or after the ammonia is introduced. The carbon dioxide may be introduced as a gas, or in powdered form, or in combination with the ammonia in the form of ammonium carbonate or bicarbonate which may be applied directly or formed in situ.
When the method of Armstrong et al. is used, many of the prior art problems of tobacco expansion are overcome. For example, the process is relatively inexpensive, has application to both tobacco stems and tobacco leaves, and greater and more permanent expansion of the tobacco is obtained. However, the Armstrong method, along with most other batch processes currently employed for tobacco expansion, is not entirely satisfactory for continuous online impregnation and expansion of tobacco due to certain requirements of these prior art batch methods.
For example, where carbon dioxide gas and ammonia vapors or gas are used as impregnants in prior art batch methods, pressure and heat build-up are created in a closed system as the result of the heat of solution of ammonia and the exothermic reaction of the ammonia, carbon dioxide and the moisture present in the tobacco in forming ammonium salts. Cooling by means of a water jacket or the like, reduced pressure, and/or agitation, is needed in order to disipate this heat build-up. If ammonium carbonate or bicarbonate is used to supply the expansion agents, it may be necessary to use reduced pressures and subject the tobacco to the ammonium carbonates in an enclosed zone for 24 to 96 hours. Alternatively, ammonium carbonate or bicarbonate may be applied to the tobacco as a dust and then held for 18 to 96 hours. A still further method involves contacting the tobacco with ammonium carbonate or bicarbonate salts suspended or partially dissolved in a suitable liquid medium for about 1 to 48 hours with the solvent being removed by exposing the tobacco to a flow of inert gas or air before puffing. The necessity for cooling means, reduced pressure or vacuum and/or long periods for impregnation or equilibration of these methods, hampers the application of such methods to continuous on-line processes.
A further problem in the conversion of existing batch impregnation processes to continuous processes is the formation of hard balls of ammonium salts within the tobacco mass and the buildup of salt deposits on the impregnator walls and in the head spaces. These difficult-to-remove salt deposits on the equipment must be removed between batches for efficient batch processing. In addition, the formation of large lumps of ammonium salts and tobacco within the tobacco mass reduces the amount of usable shredded tobacco material, reduces efficiency and increases cost.
Thus, current batch processes of tobacco expansion introduce a variety of factors which complicate and impede continuous on-line tobacco impregnation and expansion. Therefore, current batch processes do not provide a completely satisfactory means for accomplishing continuous on-line tobacco impregnation and expansion.